Using Features for Advanced Applications
3–158 TDS 684A, TDS 744A, & TDS 784A User Manual
To learn how to optimize your display of FFT data, read about how the FFT
windows data before computing the FFT math waveform. Understanding FFT
windowing can help you get more useful displays.
Windowing Process. The oscilloscope multiplies the FFT time domain record by
one of four FFT windows before it inputs the record to the FFT function. Figure
3–93 shows how the time domain record is processed.
The FFT windowing acts like a bandpass filter between the FFT time domain
record and the FFT frequency domain record. The shape of the window controls
the ability of the FFT to resolve (separate) the frequencies and to accurately
measure the amplitude of those frequencies.
Selecting a Window. You can select your window to provide better frequency
resolution at the expense of better amplitude measurement accuracy in your FFT,
better amplitude accuracy over frequency resolution, or to provide a compromise
between both. You can choose from these four windows: Rectangular, Hamming,
Hanning, and Blackman-Harris.
In step 8 (page 3–147) in To Create an FFT, the four windows are listed in order
according to their ability to resolve frequencies versus their ability to accurately
measure the amplitude of those frequencies. The list indicates that the ability of a
given window to resolve a frequency is inversely proportional to its ability to
accurately measure the amplitude of that frequency. In general, then, choose a
window that can just resolve between the frequencies you want to measure. That
way, you will have the best amplitude accuracy and leakage elimination while
still separating the frequencies.
FFT Windows